a. Field of the Invention
The present invention pertains generally to data storage and more specifically to a data storage format and method that preserves data integrity in the event of failure of two storage domains.
b. Description of the Background
Data storage systems can comprise an array of disc drives connected to one or more disc array controllers using one or more buses. Disc array controllers may be connected to one or more host systems using one or more buses. Data storage formats, such as RAID (Redundant Array of Independent Discs), may be employed to distribute user data and redundant information across multiple drives such that if a drive fails, user data may be copied, regenerated, or reconstructed (regenerated and copied to another drive) from remaining operating drives.
RAID levels include RAID-0, RAID-1, RAID-5 and RAID-6. RAID-0 provides no data redundancy. RAID-1 provides mirrored copies of stored data. RAID-5 employs a parity data block calculated from a plurality of data blocks. RAID-6 employs two-level parity such that data may be reconstructed in the event of two drive failures.
Each of the RAID levels exhibit various tradeoffs in terms of the amount of disc space employed to store data and the rate at which data may be written and read. RAID levels such as RAID-5 and RAID-6 may employ read-modify-write operations to write data that occupies less than the number of data blocks comprising the group of data block employed to calculate parity.
The organization of data within a storage array relative to each RAID level also affects read and write performance. Various data organizations may exhibit different data densities (amount of disc space utilized to store a data set) and may exhibit different read and write performance. Further, system performance may be affected by the number and complexity of calculations performed to read, write, or recover data.